80-07 Chapter 1339 Storm Water Management

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in OAC Chapter 3745-1. The WQv shall be equivalent to the volume of runoff from a 0.75-inch rainfall. (c) Runoff Rate. The peak runoff rate from the development area shall not be greater after development than it was before development. The applicant shall provide calculations proving no increase in the runoff rates from the one (n two (2), five (5), ten (10), twenty-five (25), fifty (50) and one hundred (100) year storms. (d) Runoff Volume. Increases in the runoff volume shall be offset by further restricting runoff rates. Where the land use will be mixed, the runoff coefficient should be calculated using a weighted average. Based on the increase in runoff volume, the applicant shall determine the critical storm for the development area. The runoff rate from the critical storm shall be restricted to the one (1) year pre-development storm runoff rate. The critical storm shall be calculated as follows: (1) Determine the total volume of runoff from a one-year frequency, twenty- four hour storm, occurring on the development area be.fore and after development. (2) From the volumes in paragraph (A) determine the percent of increase in volume of runoff due to development and, using this percentage, select the critical storm from this table: Table 4: Critical Storm Selection Equal To Or And Less Than Greater Than o 10 10 20 20 50 50 100 100 250 250 500 500 The 24-Hour "Critical Storm" For Dischar e is 1 Year 2 Years 5 Years 10 Years 25 Years 50 Years 100 Years (e) D!'!tention Or Retention Basin Exemption For Redevelopment Or For Expansion Of Existing Facilities. (1) For any development regulated by this chapter, the construction of a detention or retention basin may not be required for the development if the post-development peak discharge for a 100 year frequency 24 hour 27